1. Field of the Invention
The present invention relates to a wireless power feeder, a wireless power receiver, and a wireless power transmission system for transmitting power by a non-contact method.
2. Related Background Art
Japanese Patent Application Publication No. 2010-166693 discloses a wireless power transmission system which transmits power by a non-contact (wireless) method from a wireless power feeder to a wireless power receiver. In this wireless power transmission system, power is transmitted by using an electromagnetic induction effect. Therefore, the wireless power feeder 10 comprises a power feed resonance circuit having a power feed coil 14 and a power feed capacitor 15, and furthermore, the wireless power receiver 20 also comprises a power receive resonance circuit having a power receive coil 21 and a power receive capacitor 22. The power feed coil 14 and the power receive coil 21 constitute a power feed transformer Tf, and power transmission is carried out by a non-contact method using an electromagnetic induction effect between the coils.
In a wireless power transmission system which uses an electromagnetic induction effect in this way, power transmission is performed efficiently with a power factor close to “1” for the transmitted power. However, if the relative distance between the power feed coil 14 and the power receive coil 21 changes, then the mutual inductance of the coils changes and the power factor of the transmitted power declines. As a result of this, it becomes impossible to carry out efficient power transmission.
In respect of this point, Japanese Patent Application Publication No. 2010-166693 discloses an invention which finds the frequency of a resonance current of a power feed resonance circuit that corresponds to change in the mutual inductance of a power feed transformer, and sets the frequency of the voltage supplied to the power feed resonance circuit to a value corresponding to this frequency of the resonance current. For this reason, the wireless power feeder 10 has a frequency control circuit 31 in the controller 30 which performs PWM control of a voltage type inverter 13 which is connected to the power feed resonance circuit. The frequency control circuit 31 detects the output current of the voltage type inverter 13 and controls the phase frequency of the output voltage of the voltage type inverter 13, in such a manner that the power factor of the output power of the voltage type inverter 13 becomes 1. More specifically, the frequency control circuit 31 controls the voltage type inverter 13 so as to carry out zero current switching.
In recent years, a wireless power transmission system for transmitting power by a non-contact (wireless) method has been proposed, in which power is transmitted by using a magnetic field resonance effect between a wireless power feeder and a wireless power receiver. In this wireless power transmission system, the wireless power feeder comprises a power feed resonance circuit having a power feed coil and a power feed capacitor, and the wireless power receiver also comprises a power receive resonance circuit having a power receive coil and a power receive capacitor. Power transmission is performed by a non-contact method between the wireless power feeder and the wireless power receiver, by using this magnetic field resonance effect.